A motor, generator, gearbox, turbine, impeller, and other devices having (or attached to) rotating or translating shafts may be termed an “electric machine,” or collectively termed “electric machinery.” Referring to the exemplary electric motor/generator and bearings shown in FIGS. 1A and 1B, electric machine 10 is often connected to an electrical source or load via power electronics 35. Power electronics 35 include switching amplifiers, whose voltages and currents possess high frequency harmonics in addition to the fundamental component. For example, in a variable frequency drive, the fundamental electrical frequency may be 60 Hz, the switching frequency may be 10 kHz, and on/off transition time for the power semiconductors may be 1 μs to 100 ns (1-10 MHz). The higher frequency components, i.e. those above the fundamental, introduce voltages and currents throughout the machine 10 due to parasitic coupling. Parasitic coupling refers to non-desirable yet unavoidable mutual capacitances and inductances between different parts of a machine 10. The influence of this coupling is generally insignificant at the fundamental frequency but is readily apparent at high frequencies. For example, the switching voltage harmonics of a variable frequency drive may interact with the parasitic elements of the electric motor/generator 10 to which it is connected, producing high frequency electric currents that flow through these parasitic pathways, such as through the electrical capacitance of the bearings 50 that support the induction rotor 15 within the stator (i.e., non-moving components that may include a stator frame 25 and stator windings 30). With substantial levels of current in the bearings 50, a substantial high frequency potential difference between the stator 25, 30 and rotor 15 may be established, producing destructive arcs within the bearing 50 itself, as well as producing electromagnetic interference (EMI) from the shaft 20. These high frequency parasitic currents will herein be referred to as “bearing currents,” and electromagnetic interference will refer to the interference conducted or radiated by the shaft 20 of the machine 10.
Bearing currents are undesirable due to the adverse wear they cause. In ball bearings 50, for example, the inner raceways 55 and the outer raceways 65, in addition to the balls 60, experience pitting due to the high-current electric pulses that flow through them during an arcing event. During shaft rotation, bearing grease at contact points 70, 75 forms a layer of electrical insulation between the electrically conductive balls 60 and races 55, 65, forming a capacitor. This is represented by variable capacitor CB in FIG. 1B. When electric current flows through this capacitor, electric potentials develop across the bearing surfaces and can arc, causing pitting. Pitting of the bearing surfaces causes mechanical wear and premature failure of the bearing 50. Bearing failure is catastrophic for electric machinery, and failure will often propagate to systems attached to the machine 10, such as gearboxes, turbines, impellers, etc. To prevent such failures, bearing currents must be mitigated.
Electromagnetic interference may also result from parasitic coupling due to the parasitic voltage on the shaft. Existing solutions largely fall into three categories: 1) use of a brush in contact with the rotor (i.e., a non-stationary part of a machine); 2) bearing insulators; 3) electrical line filters for high frequency harmonics. First, regarding use of a brush, a brush forms a sliding contact on the rotor 15 of the machine 10. This contact exhibits a lower electrical impedance than the bearings 50, and thus bearing currents are shunted around the bearings 50, and the shaft voltage is driven towards zero. However, brushes experience significant mechanical wear and accumulate brush dust over time, requiring periodic maintenance. Second, regarding bearing insulators, methods to isolate the electrical path that bearing currents flow through typically include insulated bearing mounts or bearings with insulating balls. Structurally identical to steel bearings, ceramic bearings use ceramic balls and/or raceways rather than steel to support a rotating shaft. The use of a ceramic material makes the bearing electrically insulating, preventing the flow of currents through them. However, drawbacks of this system include the higher purchase price for ceramic bearings, and the need for significant downtime/effort to undertake the difficult task of retrofitting existing/deployed systems. And third, regarding line filters, an electrical line filter may be placed on the terminals of the electric machine 10. These filters use inductive, capacitive, and/or resistive elements to form a network that dissipates high frequency content before it can flow through the electric machine 10. Line filters can be expensive and bulky, deterring their deployment in many applications. Additionally, line filters are most often passive (i.e., have constituent components with fixed values), thereby limiting the adaptability of line filters in a changing environment.
What is needed is a device and method that will simultaneously reduce or mitigate bearing currents and electromagnetic interference without the above drawbacks, such as the need for periodic maintenance, higher costs, and limited adaptability.